The earliest surviving Christian art was not gold cruicifixes or silver chalices. Nor were they elegant triptych altarpieces. Painting being the quickest, and the most easily accomplished of the ancient artforms, it was natural that the early Christians should turn to it first to express their faith. These painted works of art were not wooden panels but fresco paintings, not on the ceilings of cathedrals but the humble arches and lunettes of Roman catacombs. Carved from a soft, porous stone, a virtural MAZE of these burial chambers hid beneath the seven legendary hills of the city of Rome. Within them were the remains of over six MILLION former citizens of the city.
Often small chapels, some no more than thirty feet square, had been carved for the worship of the dead in ancient times. Early Christians found these chapels convenient for their secret meeting places and over them, were executed some of the earliest crude, Christian paintings. Strangely these earliest Christian images were not the crucifixions, or nativities, or martyrdoms we are now accustomed to associating with religious art. Instead, the image of the "Good Shepherd" or iconography involving fish and Biblical parables were the most common subject matter employed by these largely unskilled, early Christian painters.
The frescos of the catacomb of Saints Pietro and Marcellino are strikingly beautiful and touching in their own primitive way, sharing a style that was realisitic, and reminiscent of ancient Greek and Roman images. Most, however, were not as finished or as well done. Most dealt only with early (often secret) Christian symbols or icons. Most did not feature human or divine figures. It's quite likely their artists were somewhat uncomfortable working amongst the dead in such superstitious times, and equally as likely that they desired to spend no more time than absolutely necessary breathing in the fetid stench of all that decaying flesh. One would have to say Michelangelo was fortunate that by the 1500's, Christianity was no longer literally an "undergroud" religion.